1. Field of the Invention
The present invention relates to base drive circuits of transistors, and more specifically to a base drive circuit of transistor wherein power to supply 0N base current to a transistor is made as small as possible.
2. Description of the Prior Art
A base drive circuit of transistor as an example in the prior art is disclosed in Technical Report of the Institute of Electrical Engineers of Japan (Part II) No. 162, FIG. 3.19. The prior art described in the reference as shown in FIG. 1 broadly comprises a main transistor 1, a base drive circuit 2 for driving the main transistor 1, and a signal generator 6 for giving ON/OFF command to the base drive circuit 2. The base drive circuit 2 comprises an ON base current supply circuit 3 for supplying ON base current to the main transistor 1, an OFF base current supply circuit 4 for supplying OFF base current to the main transistor 1, and an amplifier 5 for amplifying the ON/OFF command to the main transistor 1. The ON base current supply circuit 3 and the OFF base current supply circuit 4 are provided with transistors 3a, 4a rendered on or off alternately according to command of the amplifier 5, resistors 3b, 4b for limiting the ON current and the OFF current of the transistors 3a, 4a, and power sources 3c, 4c for supplying the ON current and the OFF current to the circuits 3, 4, respectively.
Next, operation of the base drive circuit shown in FIG. 1 will be described referring to FIGS. 2(a) to (f) showing operation waveform of each part of the circuit in FIG. 1.
At time T.sub.1, if signal Pon for rendering the main transistor 1 on is supplied from the signal generator 6 to the amplifier 5 as shown in FIG. 2(a), since the amplifier 5 supplies on base current i.sub.1 to the transistor 3a as shown in FIG. 2(b), the transistor 3a of the 0N base current supply circuit 3 is rendered on by the 0N base current i.sub.1. Thereby the base of the main transistor 1 is supplied with current EQU i.sub.B =(E.sub.1 -V.sub.BE)/R.sub.1 ( 1)
determined by the resistor 3b (resistance value R.sub.1) and the power source 3c (voltage E.sub.1) and forward voltage V.sub.BE between base and emitter of the main transistor 1 so as to render the main transistor 1 on.
At time T.sub.2, if the ON command Pon becomes L, since output of the amplifier 5 supplies current so that the transistor 4a is turned on and the transistor 3a is turned off, the base of the main transistor 1 is supplied with current EQU i.sub.B =-E.sub.2 /R.sub.2
(where E.sub.2 : source voltage of 4c, R.sub.2 : resistance value of 4b). Thereby storage carrier of the main transistor 1 is lost and the main transistor 1 is rendered off at time T.sub.3. After the time T.sub.3, current determined by equivalent resistance between base and emitter of the main transistor 1 flows.
Since the base drive circuit of the prior art is constituted as above described, in order to render the main transistor on, if base current as specified by equation (1) is supplied without depending on variation of base emitter voltage of the main transistor (usually, V.sub.BE =about 1-3V in Darlington transistor), a power source considerably higher than the maximum of V.sub.BE is required. If the power source of E.sub.1 =7 V is used to supply the base current i.sub.B =3 A and the main transistor is operated at a condition of conduction rate 50%, a large power of 1/2.times.(b 7 V.times.3 A)=10.5W is required only to supply the ON base current. Accordingly, the power source for the base drive circuit must be of large capacity when a large base current must be supplied to the transistor or when the number of transistors is increased.